1. Field of the Invention
The present invention relates to an exposure apparatus and method.
2. Description of the Related Art
A conventional projection exposure apparatus projects a circuit pattern of a reticle (mask) onto a wafer or another substrate via a projection optical system in manufacturing fine semiconductor devices, such as a semiconductor memory and a logic circuit, using the photolithography technology.
The minimum critical dimension or a resolution transferable by the projection exposure apparatus is proportionate to a wavelength of the light used for exposure, and inversely proportionate to the NA of the projection optical system. The shorter the wavelength is, the smaller the resolution is. Along with the recent demands for fine processing to a semiconductor device, use of a shorter wavelength of the exposure light is promoted, such as a ultra-pressure mercury lamp (such as an i-line with a wavelength of about 365 nm), a KrF excimer laser with a wavelength of about 248 nm, and an ArF excimer laser with a wavelength of about 193 nm.
However, a semiconductor device becomes rapidly finer, the lithography using the ultraviolet (“UV”) light has a limit. Accordingly, an exposure apparatus that uses the extreme ultraviolet (“EUV”) light having a wavelength of about 10 nm to about 15 nm smaller than the wavelength of the UV light, which is referred to as an EUV exposure apparatus, is being developed.
Since the energy attenuation in gasses is very high in the wavelength range of the EUV light, the optical system of the EUV exposure apparatus is arranged in a vacuum atmosphere. Photochemical reactions between oxygen in air and impurities cause contaminations, such as hydrocarbons, to optical elements, and thus the partial pressure of the hydrocarbon in the EUV exposure apparatus needs to be made small. In particular, a space that accommodates the projection optical system, which is referred to as a “projection optical system space (“POSS”)” hereinafter, needs to maintain very low partial pressure of the hydrocarbon.
On the other hand, in order to prevent inflows of the hydrocarbons into the POSS, Japanese Domestic Publication No. 2002-529927 proposes a EUV exposure apparatus that isolates the POSS from the surrounding space, such as a space that accommodates the stage. This EUV exposure apparatus maintains the pressure in the POSS higher than the surrounding pressure, preventing the inflows of the hydrocarbons from the stage space.
In order to maintain the pressure in the POSS higher than the surrounding pressure, a gas needs to be supplied to the POSS. Conceivably, the gas is supplied from the outside of the apparatus through a pipe and a nozzle.
Although it is generally conceivable that the temperature of the gas introduced from an air pressure space to the vacuum space drops due to the adiabatic expansion, it is confirmed from a simulation that when the gas is supplied into the POSS, the supplied gas's temperature rises. When the gas is supplied from the air pressure state to the POSS below 100 Pa, the gas flows in the POSS at a significant speed even with an orifice and a stop. Thereby, the supplied gas's temperature conceivably rises.
The POSS is exhausted at a constant speed. A certain amount of gas is also supplied through the pipe and nozzle. After a certain time period elapses, the gas flow reaches an equilibrium state. When a one-dimensional phenomenon is assumed for a simple description, Equation 1 below expresses the energy conservation law in the steady adiabatic flow, where h is entropy, Cp is specific heat, T is absolute temperature, and u is a gas speed.h+½×u2=const  Equation 1
Equation 2 below expresses a relationship between the entropy and the specific heat.h=Cp×T  Equation 2
Equation 3 below defines a relationship between the gas's temperature and speed from Equations 1 and 2:T1+u2/(2Cp)=const  Equation 3
Equation 3 indicates that the temperature rises as the speed lowers where the gas that has a high speed in the pipe is supplied to the POSS. When the gas whose temperature has risen flows in the POSS, a mirror position fluctuates due to the temperature rise of the optical element and the thermal expansion of a holder (structural member), degrading the pattern transfer precision.
In addition, the temperature of the supplied gas rises as the gas is supplied to the load lock chamber so as to convert its internal pressure to the air pressure. The load lock chamber is configured to feed a wafer to the exposure apparatus that is maintained in the vacuum state.
It is assumed that P denotes pressure, T denotes temperature, γ denotes a ratio of specific heat, T0(° C.) is the reference temperature of the POSS, 1 denotes an initial state in the chamber (at a low pressure state), 2 denotes a state after the gas is supplied to the chamber, and the kinetic energy of the inflow gas is assumed to be smaller than the gas's enthalpy. Then, a relationship indicated by Equation 4 below is established:T2=γ×P2×T1×T0/((P2−P1)×T1+γ×P1×T0)  Equation 4
For example, when T0=T1=296K, P2=0.1 MPa, P1=100 Pa, γ=1.4, the temperature in the gas becomes 414 K in the apparatus. This result is derived from a calculation under various assumptions, but it is known that the temperature rises in the actual experiment.
When the temperature rises in opening the load lock chamber to the air pressure, the wafer is fed while the temperature of the load lock chamber is high, and it takes a long time to stabilize the temperature of the wafer to the reference temperature.